This invention relates to a pressure sensor which measures pressure. More particularly, this invention relates to a pressure sensor which measures calories deprived from a heating element included in a thermal pressure detector or a section heated by the heating element by a diaphragm which is arranged to be opposite to and spaced from the heating element or the heated section by a fixed distance, by using the thermal pressure detector.
A pressure sensor which measures the flexible quantity of a diaphragm using an almost univocal functional relationship held between the pressure of a measurement target fluid and the flexible quantity of the diaphragm on a cylinder which diaphragm receives pressure from the measurement target fluid and using a distortion gauge which is formed on the diaphragm by a film formation technique, a photo-engraving technique or the like, and which sensor thereby obtains the pressure of the fluid proportional to the flexible quantity of the diaphragm, is widely utilized for the detection of the quantity of the intake air of an internal combustion engine, that of the oil pressure of a vehicle brake or the like.
FIG. 5 is a cross-sectional view of a conventional pressure sensor which is disclosed by, for example, Japanese Utility Model Application Laid-Open No. S61-137242 (microfilm of Japanese Utility Model Application No. S60-19572).
In FIG. 5, a reference symbol 101 denotes a metallic cylinder, 102 denotes a semiconductor monocrystalline plate which is provided. with a distortion gauge 103 and the semiconductor monocrystalline plate consists of, for example, a silicon substrate. For the pressure sensor in which the semiconductor monocrystalline plate 102 is bonded to the metallic cylinder 101 shown in FIG. 5, the metallic cylinder 101 and the semiconductor monocrystalline plate 102 differ in material and a distortion, therefore, tends to occur to the semiconductor monocrystalline plate 102 which constitutes a diaphragm due to the difference in the coefficient of linear expansion between the cylinder 101 and the plate 102 at a time when temperature change. This causes a measurement error. In addition, since the pressure of a measurement target fluid is directly applied to the semiconductor monocrystalline plate 102, it is necessary to secure sufficiently high bonding strength between the metallic cylinder 101 and the semiconductor monocrystalline plate 102.
In FIG. 6, a reference symbol 104 denotes a metallic cylinder which consists of a cut-off pipe such as a stainless pipe. A metallic thin film 105 which is welded to the cylinder 104 is formed out of a thin plate of a rolled material and it has a uniform film thickness and a flat surface because of the rolled material. The material of the metallic thin film 105 is the same as that of the cylinder 104. In addition, a silicon oxide thin film 106 which functions as an insulating film, is formed on the upper surface of the metallic thin film 105. A plasma CVD method is used to form the silicon oxide thin film 106. A silicon thin film which constitutes a distortion gauge 107 is then formed on the silicon oxide thin film by the plasma CVD method. This silicon thin film is etched to partially leave the silicon thin film and remove the other sections as shown in FIG. 6, and the distortion gauge 107 is formed out of the silicon thin film which is thus left. Furthermore, metal such as gold is deposited on the distortion gauge 107 to thereby form an electrode. A lead wire is bonded to this electrode by ultrasonic bonding. The electrode and the lead wire are appropriately connected to each other, whereby a circuit can be formed.
Each of the conventional pressure sensors shown in FIGS. 5 and 6 uses the distortion gauge. The diaphragm is distorted by the pressure of the measurement target fluid applied to the diaphragm and each of these pressure sensors measures the, distortion by the distortion gauge provided on the diaphragm. Besides these pressure sensors, a pressure sensor which detects the flexure of a diaphragm as a capacitance change is also used.
FIG. 7(a) is a cross-sectional view and FIG. 7(b) and FIG. 7(c) are top views of a conventional pressure sensor of a capacitance detection type disclosed in, for example, Japanese Patent Application Laid-Open No. S60-56233.
In FIG. 7(a), reference symbol 108 denotes a substrate which has an electrode 109 provided in the central portion of an upper surface thereof, an electrode 110 for correction concentric to the both and provided on an edge section thereof and a hole 111 provided in between the electrode 109 and the correction electrode 110. Reference symbol 112 denotes a diaphragm on one surface of which an electrode 113 is provided at a position opposite to the electrode 109. Reference symbol 114 denotes gap adjustment glass beads which are interposed between the substrate 108 and the diaphragm 112 so as to form a gap 115 between the electrodes 109 and 113. In this pressure sensor, when pressure P is applied to the diaphragm 112, the gap 115 in the central section becomes smaller and static capacitance increases between the electrodes 109 and 113. The pressure sensor is intended to measure the pressure using an almost univocal functional relationship held between this capacitance change and the pressure of a measurement target fluid.
According to the conventional pressure sensor which is constituted as explained above, when the distortion gauge formed on the silicon substrate is used, it is impossible to secure sufficient bonding strength between the cylinder and the silicon substrate on which the distortion gauge is formed. It is, therefore, impossible to directly apply the pressure of the measurement target fluid to the silicon substrate and to measure the pressure of the measurement target fluid. Accordingly, it is required to cause pressure to be acted on a buffer in a different chamber using the diaphragm which is deformed by the measurement target fluid and to measure the pressure of the buffer using the distortion gauge provided on the silicon substrate.
Further, when the distortion gauge of the silicon thin film provided on the metallic diaphragm is used, it is not easy to directly manufacture silicon thin film distortion gauges on the metallic diaphragmwhich receives the pressure in block in large quantities. This is because an apparatus for the silicon substrate (for silicon process) cannot be used as a diversion.
Moreover, according to the capacitance detection type pressure sensor, it is necessary to form an insulating layer on the metallic diaphragm and to then form a capacitance detection electrode using the photo-engraving technique or the like. As can be seen, when the metallic diaphragm is used, it is conventionally necessary to subject the metallic diaphragm to film formation processing, photo-engraving processing and the like. However, a film formation apparatus and a photo-engraving apparatus conventionally used for a silicon substrate cannot be used to carry out these processing. Besides, when the silicon substrate is used, the structure of the pressure sensor is complicated, making it disadvantageously impossible to manufacture a highly reliable, inexpensive pressure sensor.
This invention has been achieved solve the above-explained conventional disadvantages. It is an object of this invention to provide a simple thermal pressure sensor. The thermal pressure sensor thermally detects the displacement quantity of a diaphragm which receives pressure and measures a change in calorie following the deformation of the diaphragm and deprived of from the heating element of a detector or a section heated by the heating element which is away from the diaphragm by a fixed distance.
According to this invention, it is possible to manufacture measurement target elements on a silicon substrate in block in large quantities using a manufacturing technique and a manufacturing apparatus which are conventionally adapted to the silicon substrate. A metallic diaphragm provided on a cylinder is used as a pressure receiving member. It is an object of this invention to obtain a highly reliable, inexpensive pressure sensor which is not required to process the metallic diaphragm and provide a different chamber in which a buffer is held because an external force does not directly act on the measurement target element during pressure measurement.
According to one aspect of the invention, there is provided a pressure sensor comprising a diaphragm structure having at least one diaphragm, the diaphragm having first and second surfaces, the first surface of the diaphragm receiving pressure, a first thermal detection unit having a heating unit and arranged to be opposed to a central section of the second surface of the diaphragm, the first thermal detection unit detecting an amount of displacement of the diaphragm caused by a pressure change based on thermal detection, a first constant-current source which supplies a constant-current to the first thermal detection unit, a second thermal detection unit having a heating unit, wherein the second thermal detection unit positioned near and opposite to a periphery of the second surface of the diaphragm, the second thermal detection unit detecting an amount of displacement of the diaphragm caused by a pressure change based on thermal detection, a second constant-current source which supplies a constant-current to the second thermal detection unit, and a differential amplifier which receives the result of detections by the first thermal detection unit and the second thermal detection unit. The pressure sensor of the present invention is constituted so that the two thermal detection sections thermally detect the displacement quantity of the diaphragm due to pressure change as explained above. It is, therefore, unnecessary to subject a diaphragm surface which receives the pressure to processing such as film formation and photo-engraving. Thus, it is possible to manufacture the principal parts of a thermal pressure detector in block and in large quantities on a silicon substrate at simple manufacturing steps, to improve the accuracy and reliability of the thermal pressure detector, and to obtain an inexpensive pressure sensor. In addition, the pressure is thermally measured in a non-contact manner. Therefore, an external force does not directly act on the thermal pressure detector during measurement. It is unnecessary to secure sufficient bonding strength to resist the pressure of a measurement target fluid between a cylinder and the thermal pressure detector. It is possible to be of a simple structure and to obtain an inexpensive pressure sensor. Furthermore, since the second thermal detection section which is arranged to be opposed to the end section of the second surface of the diaphragm is provided, that is, since the second thermal detection section is provided outside of the diaphragm which has no displacement due to pressure, constant-current sources are connected to the first thermal detection section and the second thermal detection section, respectively and the difference between the signal output and the reference output is obtained so that a pressure signal output and a reference output which does not change according to the pressure can be obtained. It is, therefore, possible to remove a noise component in the same phase or an offset drift component caused by a change in atmospheric temperature, and to obtain an inexpensive, highly reliable pressure sensor.
In addition, the pressure sensor according to the present invention is provided with a circuit which measures the atmospheric temperature and the temperature of the diaphragm from the reference output of the second thermal detection section. It is, therefore, possible to take out a temperature measurement signal without using a special temperature measurement detector.
Moreover, the pressure sensor according to the present invention can remove the drifts of offset and sensitivity according to a change in use environment temperature or the temperature of the diaphragm using the temperature measurement signal from the circuit which measures the atmospheric temperature and the temperature of the diaphragm from the reference output of the second thermal detection section. It is, therefore, possible to obtain a highly reliable pressure sensor.